This invention relates to restoration of transmission systems and, more particularly, to restoration of optical transmission systems.
Optical transmission systems and, especially, those employing Dense Wavelength Division Multiplexing (DWDM) are desirable because they provide extremely wide bandwidths for communications channels. Each communications channel in the DWDM transmission system carries a plurality, for example, 16, 40 or even 80, optical channels (wavelengths) on a single optical fiber and single optical repeater. However, there is a trade off between providing wider bandwidth communications channels, with their corresponding lower cost of transport, and their vulnerability to large-scale disruption of communications services because of transmission medium failure. Therefore, the ability of an optical transmission system, for example, those employing DWDM, to restore itself after a transmission medium failure is very important because of its wider impact on communications services. The DWDM optical transmission systems are of particular interest because of their restoration capabilities.
Prior attempts at providing adequate restoration in optical transmission systems have focused on so-called 1+1 optical protection switching and on optical cross connect systems. The 1+1 optical protection switching is limited in its application and does not efficiently use optical fiber. Known optical cross connect systems, require the use of a relatively large optical switching fabric to accommodate the capacity of the optical transmission system. Unfortunately, current technology may not support providing such a large switching fabric having an acceptable optical performance level. Moreover, use of such a large switching fabric in the optical cross connect comes with a relatively high cost. Furthermore, the optical cross connect system will be slower in terms of restoration speed than provided by prior known SONET/SDH ring transmission systems. In order to protect all wavelengths used in the optical transmission system the prior arrangements had to switch one wavelength at a time. Such switching is very inefficient.
These problems and other limitations of prior known optical restoration systems are overcome in an optical communications system that employs a plurality of optical nodes interconnected in an optical ring transmission configuration by at least two optical transmission media, for example, optical fiber. The at least two optical transmission media, in this example, provide optical service transmission capacity and optical protection transmission capacity. Efficient restoration of optical communications between optical nodes in the ring, after an optical transmission media failure, is realized by employing a relatively simple and efficient optical switch matrix having a first number of possible switching states and, then, by utilizing only a second number of the switching states fewer than the first number to switch optically from the optical service transmission capacity of the failed or faulted optical transmission media to the optical protection transmission capacity of another optical transmission media. Optical switching states of the optical switch matrix are blocked that are not actively used for switching from the active optical service capacity of the faulted optical transmission media to the standby optical protection capacity of the other optical transmission media. Use of this relatively simple optical switch matrix allows for the bulk switching of the optical wavelengths as contrasted with the one-to-one switching of the optical wavelengths used in prior arrangements.
In a preferred embodiment of the invention, each of the at least two optical transmission media provides both bi-directional optical service transmission capacity and bi-directional optical protection transmission capacity. In a specific embodiment of the invention, each optical transmission channel (wavelength) includes 50 percent bi-directional optical service transmission capacity and 50 percent bi-directional optical protection transmission capacity.
In another embodiment of the invention, one of the at least two optical transmission media provides active optical service transmission capacity and another of the optical transmission media provides standby optical protection transmission capacity.
In still another embodiment of the invention, at least four optical transmission media (optical fiber) are utilized to provide transmit and receive active optical service transmission capacity and transmit and receive standby optical protection transmission capacity. Specifically, one pair of the optical transmission media is used to provide bi-directional transmit and receive active optical service transmission capacity and another pair of the optical transmission media is used to provide the bi-directional transmit and receive standby optical protection transmission capacity.